Generally, it has been a common practice to employ tungsten lamps in apparatus requiring a relatively intense light source such as projectors, optical lens systems and similar apparatus. However, such apparatus is frequently configured in a manner which tends to develop undesired heat from such a light source and, in turn, requires expensive and cumbersome cooling devices in order to inhibit undesired overheating, distortion of the apparatus and catastrophic failure of the system.
Additionally, it is not uncommon to replace the light source each time the apparatus is used since the life expectancy of tungsten lamps used in projectors, for example, is relatively short, i.e., 10 to 20 hrs. of operational use. Obviously, such procedures are not only costly in equipment but also in replacement time as well. Thus, such apparatus and particularly the light source commonly used in such apparatus leaves much to be desired.
An improvement over the above-described tungsten lamp system is provided by a system utilizing a high intensity discharge lamp as a light source. For example, a common form of HID lamp is the high pressure metal halide discharge lamp as disclosed in U.S. Pat. No. 4,161,672. Therein is disclosed a double-ended arc tube configuration or an arc tube having electrodes sealed into diametrically opposite ends with an evacuated or gas-filled outer envelope. However, the manufacture of such double-ended structures is relatively expensive and the configuration is obviously not appropriate for use in projectors and similar optic-lens types of apparatus.
An even greater improvement in the provision of a light source for projectors and optic-lens apparatus is set forth in the single-ended metal halide discharge lamps as set forth in U.S. Pat. Nos. 4,302,699; 4,308,483; 4,320,322; 4,321,501 and 4,321,504. All of the above-mentioned patents disclose structure and/or fill variations which are suitable to particular applications. However, any one or all of the above-mentioned embodiments leave something to be desired insofar as arc stability and minimal color separation capabilities are concerned.